Acoustically scanned image display device

ABSTRACT

An image display device features a generally thin envelope having a transparent viewing wall. A luminescent screen is disposed on the inner side of the viewing wall. Spaced within the envelope from the screen is a sheet of piezoelectric material which propagates acoustic wave s repetitively launched therein. Distributed over the piezoelectric sheet are an array of electron sources. These may be either sources for generating electrons from the vicinity of the surface of the sheet or a plurality of apertures in the sheet and through which a flow of electrons is controlled. In any case, the release of electrons toward the screen is in response to electric fields developed by the acoustic waves. An electron accelerating field is created between the sources and the screen, and the apparatus further includes an arrangement for modulating the intensity of the released electron stream in response to a video signal.

ilniteel States Patent 91 Adler et al.

[ Apr. 17, 1973 1 ACQUSTICALLY SCANNED IMAGE DISPLAY DEVICE [75] Inventors: Robert Adler, Northfield; Sam H.

Kaplan, Chicago, both of 111.

[73] Assignee: Zenith Radio Corporation, Chicago,

22 Filed: Mar. 22, 1972 21 Appl. No.: 236,885

[52] US. Cl. ..l78/7.3 D, 340/166 EL, 315/169,

l78/5.4 EL, 313/108, 313/109 [51] Int. Cl. ..H0lj l/62, H0 1 j 63/04 [58] Field of Search ..178/5.4 EL, 7.3 D,

178/67; 340/166 EL; 315/169 R; 313/108 R, 108 A, 108 B, 108 D, 108 E, 109, 109.5

Primary ExaminerGareth D. Shaw Attorney-John H. Coult et al.

[57] ABSTRACT An image display device features a generally thin envelope having a transparent viewing wall. A luminescent screen is disposed on the inner side of the viewing wall. Spaced within the envelope from the screen is a sheet of piezoelectric material which propagates acoustic wave 5 repetitively launched therein. Distributed over the piezoelectric sheet are an array of electron sources. These may be either sources for generating electrons from the vicinity of the surface of the sheet or a plurality of apertures in the sheet and through which a flow of electrons is controlled. ln any case, the release of electrons toward the screen is in response to electric fields developed by the acoustic waves. An electron accelerating field is created between the sources and the screen, and the apparatus further includes an arrangement for modulating the intensity of the released electron stream in response to a video signal.

14 Claims, 7 Drawing Figures PATENTEUAPR 1 71913 SHEU 1 [IF 2 ACOUSTICALLY SCANNED IMAGE DISPLAY DEVICE BACKGROUND OF THE INVENTION The present invention relates to image display devices. More particularly, it pertains to image display devices having an overall shape of a flat, thin character.

The type of image display device presently used for the reproduction of television images is the well-known cathode-ray or picture tube. It includes one or more electron guns for developing and projecting a beam of electrons toward a spaced luminescent screen. To allow for scanning of the screen by the electron beam, the envelope of the picture tube typically is somewhat in the shape of a funnel, the screen being disposed across the larger end. Particularly with the advent of highly miniaturized solid-state circuitry, the components in a television receiver necessary to operate the cathode-ray tube and supply it with the necessary video information now may occupy much less space in the overall cabinet than is required just for the cathoderay tube itself. That is, the total overall size of the entire television receiver could be substantially smaller than at present except for the significantly large volume of space required to accept the cathode-ray tube.

In view of the foregoing, numerous attempts have been made to reduce the volume required for the image display device in a television receiver. One general approach has been that of in some way folding or bending the electron beam toward the plane of the luminescent screen so as to reduce the depth of the envelope. However, the achievement of accurate imageregistration and other difficulties have precluded any significant degree of adoption of this approach. Numerous other suggestions have included at least mor'e-or-less flatpanel arrays of electroluminescent cells, light-emitting semi-conductor devices, particle light modulators,'gas discharge cells and liquid crystals. A different approach previously suggested is that of passing a flood of electrons produced by a photo-emitting through a matrix of channel multipliers before projection upon a luminescent screen. The individual different multipliers are activated in turn so as to produce a scanning action and all are subjected to intensity control in response to a video signal. The use of the channel multipliers, of course, may contribute to increased brightness, other things being the same. So far, however, such devices have not proven to be generally adequate by reason of one or more of lack of sufficient resolution, inadequate brightness, insufficient contract range or improper relationships between the persistence of individual picture elements and the integration requirements of the human eye.

One prior suggestion of particular interest is described by Stephen Yando in an article entitled A Solid-State Display Device" which appears in Proceedings of the IRE, December, 1962 at pages 2445-2451. In that case, a flat piezoelectric ceramic sheet is contiguous with a layer of electroluminescent material. Acoustic waves are launched'into'a pair of mutually-perpendicular edge surfaces of the sheet. At any place where the two acoustic wavefronts intersect, a high electric field intensity is produced as a result of piezoelectric action. The strength of that field is sufficient to effect the emission of light by the immediatelyadjacent portion of the electroluminescent layer. In a similar device, the electric fields developed by such piezoelectric scanning have been employed to excite light-emitting diodes. In either case, adjustment of the time as between the two different acoustic wavefronts permits the light-emitting spot to scan a succession of lines across the device. By, at the same time applying a uniformly distributed field or signal to the electroluminescent layer or other device in response to a video signal, image reproduction is obtained. However, these particular approaches have not been commercially exploited. Apparently, this again is because of such deficiencies as inadequate resolution, brightness and contrast.

It is, accordingly,a general object of the present invention to provide a new and improved image display device in which at least some of the aforenoted disadvantages and deficiencies are overcome.

It is a particular object of the present invention to provide a new and improved image display device in which the overall dimensions are such as to result in a compact or thin assembly.

Another object of the present invention is to provide such a device which may be manufactured by use of presently available techniques.

In accordance with the present invention, an image display device includes an evacuated envelope having a narrow, generally rectangular cross-section in one plane with at least one wall normal to that plane being optically transparent. Disposed on the inner side of the transparent wall is a screen that exhibits luminescence in response to electron bombardment. A sheet of piezoelectric material is spaced within the envelope from the screen and propagates acoustic waves that are repetitively launched into the sheet. Effectively distributed over the sheet is an array of electron sources arranged in a predetermined pattern. Each source effects the release of electrons toward the screen in response to electric fields developed by the acoustic waves. An electron-accelerating field is created between the sources and the screen. Finally, the intensity of the released electron stream is modulated in response to a video signal.

BRIEF DESCRIPTION OF THE DRAWINGS:

The features of the invention which are believed to be novel are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings, in the several figures of which like reference numerals identify like elements, and in which:

FIG. 1 is an overall perspective view of a portion of an image display device arranged in accordance with FIG. 6 is a schematic diagram illustrating the layout of certainccrnponents in the device of FIG. and

FIG. 7 is a schematic diagram of the equivalent circuitofthe device ofFIG. s. FIG. 1 illustrates a scanning arrangementwhich is used in each of the devices to be discussed in connection with FIGS. 2 and 5. It also is the same general type of scanning arrangement contemplated in the aforementioned article by Yando, and the article may be consulted for a perhaps more detailed discussion of both theory and structure together with the presentation of results obtained experimentally. For present purposes a somewhat briefer description will suffice.

Thus, a flat, thin poled ceramic sheet 10 of piezoelectric material, such as a lead zirconatetitanate, is generally rectangular in shape. Along one edge surface is a first input transducer 1 1 that responds to signals from a source 12 to launch acoustic waves in sheet 10 as indicated by the straight wavefront 13 which travels at a velocity V from the upper-left to the lower-right in FIG. 1. Analogously, a second input transducer 15 is disposed along another and adjacent edge surface of sheet 10. In response to a signal from a source 16, transducer 15 launches acoustic waves as indicated by straight wavefront 17 traveling also at velocity V, but in this case from the lower-left to the upper-right in FIG. 1. The paths of the two waves cross so as to define an intersection 18. In this particular case, where the paths of the two waves are at a right angle to one another, intersection 18 moves to the right at a velocity which is Viv,

Affixed to an edge surface of sheet 10 opposite transducer 11 is an acoustic wave termination 20 of an absorbing material such as lead. Similarly, another terminator or absorber 21 is affixed to the edge surface of sheet 10 opposite transducer 15. Still another terminator 23 is affixed to the outer surface of input transducer 11 by a ceramic layer 24 and serves to absorb and attenuate the acoustic waves inherently launched in an outward direction by transducer 11. In the same way, a terminator 25 is affixed to the outer surface of transducer 15 by still another layer 26 of the ceramic material. Terminator 25 once again serves to damp out waves projected outwardly by transducer 15 and of no use in the operation of the device.

In operation, the structure of FIG. 1 functions as a terminated electromechanical transmission line in which a pair of electrical input signals are converted into a pair of elastic pulses that travel through the line at right angles to one another. Because the wave propagation takes place in a piezoelectric medium, an electric field caused by direct piezoelectric effect accompanies each elastic' wave pulse. With sheet 10 having been poled in a direction normal to its major dimensions, the predominant electric field is in that same direction. By placing a probe on the surface of sheet 10 in FIG. I facing the viewer, a voltage pulse will be ob-. served each time an acoustic wavefront passes. With a metal ground plane electrode (not shown) affixed to the back side of sheet 10, the amplitude of the pulse is.

increased.

As specifically shown in FIG. 1 input voltages have I been momentarily applied to each of the two transducers at the same time. The pair of wavefronts l3 and 17 exist at respective locations equally spaced from-their v corresponding transducers. At intersection 18, the mechanical pressures involved reinforce one another and, as a result create a spot of enhanced electric field strength. By controlling the relative timing of, the two difierent signals from sources 12 and 16, the line of travel of the spot may be moved up or down in FIG. 1. In this manner, a complete image raster or series of successive lines may be caused to be traced throughout a rectangular area 27 as indicated by dashed lines in FIG. 1.

In FIG. 2, an evacuated envelope 30 is of a narrow, generally rectangular cross-section in the plane of the drawing. Envelope 30 preferably is of glass, so that at least its upper wall 31 is optically transparent. A luminescent screen 32 is disposed on the inner surface of wall 31. Screen 32 is of 'a material, such as those commonly used in present day cathode-ray tubes, which exhibits luminescence in response to electron bombardment. A sheet 34 of piezoelectric material is spaced within envelope 30 from screen 32. As illustrated, an input transducer 35 is sandwiched between one edge surface of sheet 34 and an acoustic wave absorber 36. Another absorber 37 is affixed to the opposite edge surface of sheet 34. Analogously, a second input transducer and an associated absorber are affixed to an orthagonally related edge surface (not shown) of sheet 34 and opposite still another absorber (not shown) on the remaining edge surface. As such, the construction and operation of these input transducers and sheet 34 is that of providing a controllably scanned moving spot of high electric field intensity in the same manner as already described in connection with FIG. 1.

Afiixed to and distributed over the major surface of sheet 34 facing screen 32 are a plurality of conductive electrodes 38 mutually spaced apart and arranged in a predetermined pattern. In this case, the pattern is in the form of a series of rows. Within each row, the electrodes are spaced one after the next so as to define a succession of individual picture elements in a row. Immediately overlying electrodes 38 is a film 39 of insulating material sandwiched between the electrodes and a highly conductive layer 40 which races screen 32. Finally, the structure within envelope 30 includes a conductive layer 42 on the side of sheet 34 away from screen 32; conductive layer 42 serves as a ground plane and enhances the electric field output resulting from the scanned acoustic waves by serving as a counterpole.

Completing the physical structure, each of the different ones of electrodes 38 is connected to ground plane 42 through a respective high value resistor 48 connected between the ground plane electrode or layer 42 and the corresponding one of electrodes 38. The layout for making such connections is indicated in FIG. 4 for a first row of tunnel cathodes each with an associated return resistor 48 and a second row of such resistors 48a for association with a corresponding second row of tunnel cathodes. In practice, the individual return resistors themselves and their connecting leads may be printed on the surface of sheet 34 in the spaces between electrodes 38. Alternatively, a high resistance film may fill the entire area between electrodes 38, with lower resistance strips added as required to provide a DC. g round return.

In operation, all tunnel cathodes are normally biased off by the potential from source 45. The appearance of the enhanced electric field developed at intersection 18 (FIG. 1) under any given one of the tunnel cathodes effects conduction in that cathode for a very short time interval. The degree of conduction is proportional to the video signal existing existing at that instant. With a tunnel cathode efficiency of, say, 5 percent, only 2 milliamperes of current through the diode is required to produce a current of one hundred microamperes in the vacuum between screen 32 and layer 40. With a lO-volt potential needed to operate each tunnel cathode, the control power thus is 20 milliwatts. For the case in which one-half of the control power is derived from the propagating acoustic wave, and allowing for the loss of ten percent of the acoustic power en route, the average power must be 100 milliwatts. For an example of three hundred picture elements in a single scanning line, the peak power in the short acoustic pulse thus is approximately watts, a power level easily generated with piezoelectric transducers.

In the alternative embodiment of FIG. 5, free electrons are produced at a point behind the wave propagating piezoelectric sheet, and their flow toward a luminescent screen is controlled as they pass through apertures in the sheet. Thus, an evacuated envelope 50 has an overall shape generally the same as that in the embodiment of FIG. 2. Disposed on the inner side of a transparent wall 51 is a luminescent screen 52. Spaced from the latter is a piezoelectric sheet 54 which features a plurality of apertures 55 distributed so as once again to define an array of electron sources arranged in a selected pattern corresponding to the desired lines of picture elements. Surrounding each aperture 55 on the side of sheet 54 facing away from screen 52 is a conductive ring 56. Spaced from rings 56 on the other major wall 57 of the envelope 50 is a photo-cathode 58 which responds to radiation, such as light and indicated by arrows 59, to which wall 57 is transparent. A conductive layer 60 layer 60 is disposed on the side of piezoelectric sheet 54 facing screen 52, with apertures 55 also extending through that layer. Layer 60 again serves to enhance, the strength of the voltage pulses generated by the traveling acoustic wave and also serves as a connecting element to the external apparatus.

As indicated in the equivalent circuit of FIG. 7, each ring 56 is connected to a common return lead 62 picted in FIG. 6 for a first row of rings 56 having return resistors 63 connecting to lead 62 and a second row of rings 56a associated with corresponding resistors 63a and a return lead 62a. Again, as in the case of FiG. 2, the individual resistors 63 and 630 may be replaced by a resistive film covering the entire surface between the through an individual resistor 63. Such a layout is derings with suitable low resistance strips being arranged to provide a ground return. Piezoelectric sheet 54 is represented in the equivalent circuit of FIG. 7 by its corresponding plurality of transmission line sections, each composed of a series inductor 65 and a shunt capacitor 66. A positive high voltage is connected to luminescent screen 52 so as to accelerate those electrons passing through apertures 55 as well as, by proximity focusing, to concentrate into a'well defined spot the group of electrons projected out of anyone of apertures 55. A video signal source 68 is connected in series with a bias potential source 69 with one end of that series combination being connected to large area photocathode 58 and the other end being coupled through a blocking capacitor 70 to conductive layer 60 as well as directly to common return lead 62.

In operation, piezoelectric sheet 54 serves as a storage grid with the amount of charge stored at any instant acting by way of the associated control ring 56 to control the level of electron flow permitted through the corresponding aperture 55. The storage is electrostatic and represents a recording of an image that is to be stored, for example, for one frame period. Charge is deposited on the piezoelectric sheet by the electrons from photo-cathode 58. The amount of charge deposited is a function of the video signal which is applied between the photocathode and thus formed storage grid. The charges are deposited in the region of any given one of rings 56 at the time when acoustic wave intersection 18 (FIG. 1) is under that ring so as to render it instantaneously positive. After departure of the acoustic wave pulse from the ring, the level of deposited charge controls the amount of flow of electrons from photo-cathode 58 through the corresponding aperture 55 and on to luminescent screen 52.

The amount of bias required from source 69 is somewhat dependent upon the dimensions chosen for the different elements and spacings. In some cases, the bias level required for charging (writing-on) the storage grid might, for example, be too high to permit continuous read out; in such a case, the read out and writing operations may be programmed so as to be sequential. However, by controlling the amount by which the field is permitted to penetrate through apertures 55, simultaneous write-in and display may be achieved. Such control may, in turn, be attained by adjustment of low voltage potential LV+ applied conductive layer 60.

Both embodiments have been described on the basis of the scanning approach of FIG. 1 which involves the propagation of acoustic waves in two directions. In some applications, however, an embodiment with but one dimensional acoustic scanning may be desired. In that case, any second dimension required may be produced by other means such as conventional switching between parallel conductors. That is, a commutator might be incorporated to sequence between different rows of the return leads 62 (FIG. 6) in the second embodiment. Analogously, conductive layer 40 in FIG. 2 might be segmented into a plurality of rows which would then be selected in sequence by a commutator. In the particular scanning device described by Yando in the aforenoted article, video voltage pulses are utilized to develop the scanning acoustic pulses. It is to be noted, however, that radio frequency signals may instead be used in order to obtain less dispersion at the expense of increased attenuation.

In both embodiments, the control of the electron flow involves the non-linearity required to obtain threshold action and contrast variation. Present-day techniques developed in connection with integrated circuitry and thin film devices permit formation of the tunnel cathodes with extremely minute dimensions or of apertures and control rings similarly small in size. Consequently, a large number of image elements may be formed per line with resultant high resolution. As in the case of conventional cathode-ray tubes, efficient phosphors are available for the luminescent screens, and the employment of a high accelerating voltage permits the attainment of substantial brightness. Yet, there is no need for deflecting a long electron beam such as in a conventional cathode-ray tube. Consequently, the resulting package may be comparatively thin in the direction normal to the luminescent screen.

While particular embodiments of the invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects and, therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention.-

We claim:

1. An image display device comprising:

an evacuated envelope having a narrow, generally rectangular cross-section in one plane at least one wall normal to said plane being optically transparent;

a screen disposed on the inner side of said one wall and exhibiting luminescence in response to electron bombardment;

a sheet of piezoelectric material spaced within said envelope from said screen and propagative of acoustic waves;

an array of electron sources effectively distributed oversaid sheet in a predetermined pattern with each source effecting the release of electrons toward said screen in response to electric fields developed by said acoustic waves;

means for creating an electron accelerating field between said sources and said screen;

means for modulating the intensity of said electrons in response to a video signal;

and means for repetitively launching said acoustic waves in said sheet.

2. A device as defined in claim 1 in which said launching means further comprises;

means for projecting in said sheet'a first front of acoustic waves along a first path;

means for projecting in said sheet a second front of acoustic waves along a second path that crosses said first path in said sheet;

and means for biasing said electron sources so as to effect the release of said electrons only in response to a coincidence of electric fields developed by said first and second fronts of waves.

3. A device as defined in claim 1 in which said electron sources each comprises an individual tunnel cathode rendered electron emissive in response to said electric fields, with the amount of such emission being responsive to the level of said video signal.

4. A device as defined in claim 3 in which each of together to form respective and conti uous film s.

. A device as efined in claim which includes means, including a resistive element effectively connected to respective difierent ones of said tunnel cathodes, for biasing said tunnel cathodes to emit electrons only in response to said electric fields.

7. A device as defined in claim 3 in which said resistive element includes a plurality of resistors individually connected in series with respective different ones of said tunnel cathodes.

8. A device as defined in claim 3 which includes means for coupling said video signal across the series combination of said tunnel cathodes and said piezoelectric .sheet.

9. A device as defined in claim 1 in which said electron sources are afi'ixed to the surface of said sheet facing said screen, and a conducting electrode is affixed to the opposite surface of said sheet, said conducting electrode forming a counter-pole in the application of said acoustic waves.

10. A device as defined in claim 1 in which said piezoelectric sheet includes:

a plurality of means each defining an aperture in said sheet in correspondence with each of said electron sources;

and means disposed on the side of said sheet opposite said screen and productive of electrons directed toward said apertures.

11. A device as defined in claim 10 in which:

said productive means is a layer of photo-cathodic material disposed on the inner side of another walls of said envelope opposite said one wall;

said other wall is transparent to radiation;

and said device includes means for irradiating said other wall. I

12. A device as defined in claim 10 in which said modulating means is coupled between said productive means and the side of said sheet facing away from said productive means.

13. A device as defined in claim 10 in which a plurality of rings of conductive material individually encircle respective different ones of said apertures, and said rings are each returned resistively to one side of said modulating means.

14. A device as defined in claim 10 in which a conductive electrode is affixed to the side of said sheet facing said screen and around said apertures, said conductive electrode forming a counter-pole in the application of said acoustic waves.

i i i i 

1. An image display device comprising: an evacuated envelope having a narrow, generally rectangular cross-section in one plane at least one wall normal to said plane being optically transparent; a screen disposed on the inner side of said one wall and exhibiting luminescence in response to electron bombardment; a sheet of piezoelectric material spaced within said envelope from said screen and propagative of acoustic waves; an array of electron sources effectively distributed over said sheet in a predetermined pattern with each source effecting the release of electrons toward said screen in response to electric fields developed by said acoustic waves; means for creating an electron accelerating field between said sources and said screen; means for modulating the intensity of said electrons in response to a video signal; and means for repetitively launching said acoustic waves in said sheet.
 2. A device as defined in claim 1 in which said launching means further comprises; means for projecting in said sheet a first front of acoustic waves along a first path; means for projecting in said sheet a second front of acoustic waves along a second path that crosses said first path in said sheet; and means for biasing said electron sources so as to effEct the release of said electrons only in response to a coincidence of electric fields developed by said first and second fronts of waves.
 3. A device as defined in claim 1 in which said electron sources each comprises an individual tunnel cathode rendered electron emissive in response to said electric fields, with the amount of such emission being responsive to the level of said video signal.
 4. A device as defined in claim 3 in which each of said tunnel cathodes includes a conductive electrode individually affixed to the surface of said sheet facing said screen, a conductive element facing said screen, and an insulating layer sandwiched between said element and said electrode.
 5. A device as defined in claim 4 in which said conductive elements and said insulating layers each join together to form respective and contiguous films.
 6. A device as defined in claim 3 which includes means, including a resistive element effectively connected to respective different ones of said tunnel cathodes, for biasing said tunnel cathodes to emit electrons only in response to said electric fields.
 7. A device as defined in claim 3 in which said resistive element includes a plurality of resistors individually connected in series with respective different ones of said tunnel cathodes.
 8. A device as defined in claim 3 which includes means for coupling said video signal across the series combination of said tunnel cathodes and said piezoelectric sheet.
 9. A device as defined in claim 1 in which said electron sources are affixed to the surface of said sheet facing said screen, and a conducting electrode is affixed to the opposite surface of said sheet, said conducting electrode forming a counter-pole in the application of said acoustic waves.
 10. A device as defined in claim 1 in which said piezoelectric sheet includes: a plurality of means each defining an aperture in said sheet in correspondence with each of said electron sources; and means disposed on the side of said sheet opposite said screen and productive of electrons directed toward said apertures.
 11. A device as defined in claim 10 in which: said productive means is a layer of photo-cathodic material disposed on the inner side of another walls of said envelope opposite said one wall; said other wall is transparent to radiation; and said device includes means for irradiating said other wall.
 12. A device as defined in claim 10 in which said modulating means is coupled between said productive means and the side of said sheet facing away from said productive means.
 13. A device as defined in claim 10 in which a plurality of rings of conductive material individually encircle respective different ones of said apertures, and said rings are each returned resistively to one side of said modulating means.
 14. A device as defined in claim 10 in which a conductive electrode is affixed to the side of said sheet facing said screen and around said apertures, said conductive electrode forming a counter-pole in the application of said acoustic waves. 